Just last week, I wrote a piece on the policy debates Republicans should be having among themselves, and I didn't talk about marriage at all; in the original draft, I said something about a debate on marriage still being too unlikely to expect.
I might have been wrong, but I'm still guessing that Portman will remain the exception, and that opponents of marriage will still maintain a solid veto over the presidential nomination in the 2016 cycle.
To some extent, that's because public opinion within the GOP, and presumably especially among the presidential primary electorate, still runs overwhelmingly against marriage equality. It's possible that could change, especially if GOP elite opinion continues to change, but right now it's hard to see a same-sex marriage supporter benefiting even in a large field in which staking out a minority position could have some advantages.
But the real reason I don't see it happening in 2016 is group-based. Right now, large, well-organized GOP-aligned groups strongly oppose any change in the Republican position; there's simply nothing comparable on the other side of the issue. Nor is there really likely to be anything similar.
Truth is, I'm not sure a policy debate on this is really in the GOP's interest. There's really not much to debate; it's pretty clear where the issue is going, and hard to see how a knock-down-drag-out within the party helps them (in contrast to issues such as national security in which they really need a well-thought-out position, and a nomination fight is probably a good way to get there). Nor is there any real need for Republicans to flip on the issue; it's not going to be a major voting issue, after all. No, I'd think that the best strategy for the GOP on this one is to figure out how to walk away quietly. Yes, that will involve more politicians doing what Portman did -- but it will also involve others just not talking about it, and accepting defeat when defeat comes, and then treating it as a dead issue after that.
If that's correct, then the best bet is to continue to give Christian conservatives a veto on the issue in 2016, at least -- but to otherwise keep the issue on the back burner.